1. Field of the Invention
This invention relates to speed control devices in rotative drive systems.
2. Description of the Related Art
In the rotative drive systems, a wide variety of types of control devices have been proposed. The speed control devices in the rotative drive systems generally operate in such a manner that the motor is driven on the basis of a set value of speed and at the same time the actual speed of the motor is sensed on detection of a difference from the set value to feed it back to the control signal. To further diminish the error of the actual speed against the set value, in most cases, not only the proportion term proportional to the above-described difference but also an integration term of that difference with respect to time is made to be included in the control information to be fed back. With such an arrangement, the stabilization of the drive control of the motor at constant speed is improved.
The above-described feedback servo system has found its use even in, for example, that type of the speed control device which is adapted to the still video apparatus for recording still pictures on a magnetic disc called "video floppy" disc, and reproducing them from it wherein the motor has to rotate the video floppy disc at a constant speed. head is arranged to move vertically and radially relative to the recording surface of the magnetic disc. To carry out recording or reproduction, the magnetic disc is first rotated at a constant speed and the magnetic head is then moved to come into contact with the recording surface of the magnetic disc and then carries out recording or reproducing concentrically. Further by moving the magnetic head radially, a track to be recorded or reproduced is chosen.
In the above-described still video apparatus or the like, however, the load on the motor of rotating the magnetic disc at a constant speed varies to a large extent between when the magnetic head is accessing the magnetic disc in contact relation and when the magnetic head is separated from the magnetic disc. Therefore, even though the motor for rotating the magnetic disc is provided with such a magnetic disc rotation control system as described above so that it is controlled always so as to be rotated at a constant speed, when, while the disc is in the stationary rotation state, the head is loaded onto the disc, the load of the motor changes, which in turn causes a change of the speed. Since it has no special control system that responds to this change of the load, that part of the variation of the speed which is attributable to the change of the load amounts to a great proportion. This constitutes a problem that it takes a long time to return again to the stationary speed. Another problem arises from the difference in the motor load between when the head is in contact with the inner circumference side of the disc and when the head is in contact with the outer circumference side. In this case, it is impossible to rotate the disc under the condition that the load remains the same over the entire length of the radius of the disc. Hence, a speed control means which is effective to this has been desired.
Meanwhile, in such a feedback servo system for the motor, for the purpose of swiftly effecting start-up of the motor, means may be provided for memorizing the above-described stationary error or integration term of the feedback control system and, after the start of rotation of the motor, detecting when the proportion term becomes "0", thereupon supplying the memorized value of the integration term, as disclosed in U.S. Pat. No. 4,580,084.
According to U.S. Pat. No. 5,580,084, however, all what is memorized is only that value of the integration term which is obtained at the time of the stationary rotation. On this account, that technique is difficult to be applied to, for example, the above-described still video apparatus or the like, where even the changes of the load resulting from the transition from the head load condition to the head unload condition or vice versa, and further from the variation of the relative position of the head to the disc must be taken into account. In the apparatus having such a wide range of variation of the load, therefore, no sufficient performance could not be obtained. In more detail, even if the value of the integration term obtained at the time of the stationary rotation under the usual condition, for example, of no load or a certain load is supplied, occurrence of a change of the load condition does not allow the motor to swiftly return to the stationary rotation state. In application to the still video camera or like apparatus where a fast ramp characteristic is required in view of snap shots and moreover the load differs with different radial positions of records on the disc, therefore, there is a problem that no sensible motor control can be performed.
Another example of the technique of memorizing the value of the integration term obtained at the time of the stationary rotation and applying it when the motor is rotating is shown in U.S. Pat. No. 4,816,937. Even this example has a similar problem. Also, as the servo system of this kind, mention may be made of U.S. Pat. No. 4,855,654.